'Use by' dates are for highly perishable food such as milk and yoghurt. You are advised to not consume them if they have passed their 'use by' dates.
'Best before' dates are for food with a longer shelf life such as cereals. They indicate how long the food will be at its best quality. ‘Sell by’ dates inform retailers when to take the item off the shelves.
However, the safety of a food product is not dependent on its expiry date alone. Products not stored or handled properly can be unsafe for consumption too. So remember to check for signs of spoilage before consuming them!